Gas transport in bentonite

Abstract

The gas permeability of the Spanish FEBEX bentonite compacted at dry densities of between 1.4 and 1.8 g/cm3 with high water contents was measured for different confining, injection and backpressures. The results were compared with results obtained in previous investigations for lower degrees of saturation. It was checked that gas permeability was greatly affected by dry density, decreasing about three orders of magnitude when it increased from 1.5 to 1.8 g/cm3 for similar water content. The increase of water content caused also a decrease in gas permeability. It was found that both gas permeability and the relative gas permeability were mainly related to the accessible porosity. These relationships could be fitted to potential expressions with exponents between 3 and 4, as well as the relationship between intrinsic permeability and void ratio. For gas pressures below 1.2 MPa no effect of the injection or confining pressures on the value of permeability was detected. For a given confining pressure the permeability value decreased as the effective pressure increased, especially if the increase in effective pressure was due to a decrease in gas backpressure. It was checked that the Klinkenberg effect was not significant for this material in the range of pressures applied in the tests. The gas breakthrough pressure values in FEBEX saturated bentonite were determined for different dry densities. They increased clearly with dry density and were always higher than the swelling pressure of the bentonite. In high density samples gas flow tended to stop abruptly after breakthrough, whereas in lower density samples gas flow decreased gradually until a given pressure gradient was reached. The permeabilities computed after breakthrough (which usually did not stabilise) were inversely related to dry density. This would indicate that, even if the flow took place predominantly through preferential pathways that sometimes closed quickly after breakthrough and others remained open allowing decreasing gas flow, the swelling capacity of the bentonite matrix (lower as the density is lower) had also an effect on path formation and consequently on permeability. After resaturation of the bentonite the same breakthrough pressures and permeabilities were found, pointing to the perfect healing of these preferential pathways. A sealed interface along the bentonite did not seem to affect the breakthrough pressure or permeability values.